A mountain bike’s geometry is perhaps the most important aspect of its whole design, the geometry dictating the overall handling and feel of the bike. Geometry can differ massively between different brands and models of bikes – here are some of the measurements to look out for:
- Seat angle
The angle of the seat tube relative to the ground.
A steeper angled seat tube will put the saddle directly over the bottom bracket, making pedalling easier and more efficient. This also helps with climbing, pushing your weight forwards to make sure your front wheel doesn’t lift up.
An angle around the mid-70s is best.
- ‘Effective’ top tube length
The ‘effective’ length from the top of the head tube to the seatpost, measured horizontally.
This measurement provides a better idea of how roomy a bike will feel, compared to just simply measuring the length of the top tube. It factors in stem length and the offset of the saddle to help give you an idea of how the bike will feel once you’re sat in the saddle.
- Head angle
The angle (measured from horizontal) of the steerer tube of the fork.
Head angles massively influence steering and overall handling. Slacker angles slow down a bike’s steering response, but make descending a little easier. Steeper angles make the handling feel more responsive and nippier on flat terrain.
Bikes that are built for flying downhill benefit, therefore, from slacker angles in the low 60s. Whereas XC bikes, designed for charging over off-road rolling terrain, benefit from steeper head angles in the high 60s.
- Down tube length
The distance from the centre of the bottom bracket to the centre of the bottom of the head tube.
Like the ‘effective’ top tube length, the down tube length provides an indication of how roomy the bike will feel once you’re sat in the saddle – it’s just a lot easier to measure. It’s also a great tool for working out a bike’s true size and whether it will fit you.
The horizontal distance between the front and rear axles.
The longer the wheelbase, the more stable your bike will feel. This comes at a cost however, as the longer the wheelbase, the greater your steering angle – meaning it’ll be a lot harder to skirt around tight corners on a bike with a long wheelbase.
- Bottom bracket height
The vertical distance from the floor to the centre of the bottom bracket.
Lower bottom brackets improve stability and make the bike feel a lot more agile when turning. This is because it lowers the rider’s centre of mass, giving the bike a more planted feel. The disadvantage of a low bottom bracket, however, is that it increases the chance of bashing the pedals or chainrings on the ground, so keep this in mind.
Hardtail Or Full Suspension
The type of off-road riding you’re planning on doing is going to massively influence your suspension choice.
These bikes have a rigid back end, making them a lot stiffer and more powerful over rolling trails – this is why most XC bikes are hardtails.
They have suspension solely in the front, which helps to absorb most of the buzz from the trails, but will still get hit hard by really rocky stuff. With only one suspension system, a hardtail bike is often lighter than a full suspension rig, making it the bike of choice for those looking to race over variable terrain.
These bikes have suspension in both the front and rear end of the bikes, offering increased comfort and control over rough and gnarly trails.
They’re more suited to technically demanding terrain, particularly downhills which require a large amount of suspension just to absorb all the big hits and jumps. They may be a little more complicated than hardtails, and therefore harder to maintain, but they do offer significant performance advantages on the more technical trails.